1. Field
The present invention relates to a light emitting diode and a method of manufacturing the same and, more particularly, to a light emitting diode having improved light extraction efficiency and a method of manufacturing the same.
2. Discussion of the Background
Generally, group-III nitrides such as gallium nitride (GaN) have excellent thermal stability and direct transition type energy band structure, and thus have attracted attention as materials for light emitting devices for emitting light in visible and ultraviolet bands. In particular, blue and green light emitting devices using indium gallium nitride (InGaN) have been used in a wide application range, such as large natural color flat displays, signal lamps, indoor lighting, high flux light sources, high resolution output systems, optical communications, and the like.
A light emitting diode includes an n-type semiconductor layer, a p-type semiconductor layer and an active layer interposed between the n-type and p-type semiconductor layers, and emits light through recombination of electrons and holes injected into the active layer upon application of forward bias to the n-type and p-type semiconductor layers.
Since it is difficult to produce a homogeneous substrate capable of growing gallium nitride layers, a group-III nitride semiconductor layer is generally grown on a heterogeneous substrate having a similar crystal structure to the gallium nitride layers through metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). As for the heterogeneous substrate, a sapphire substrate having a hexagonal crystal system is generally used.
However, an epitaxial layer grown on the heterogeneous substrate has a relatively high dislocation density due to lattice mismatch and difference in thermal expansion coefficient between the epitaxial layer and the growth substrate, thereby providing a limit in improvement in luminous efficacy of the light emitting diode.
Accordingly, studies have been made to develop a novel technology capable of producing a gallium nitride-based light emitting diode using a gallium nitride substrate as a growth substrate. Since the gallium nitride substrate is a homogeneous substrate with respect to epitaxial layers grown thereon, the gallium nitride substrate can improve luminous efficacy of the light emitting diode by reducing crystal defects in the epitaxial layers.
For a conventional sapphire substrate, a certain pattern is formed on the growth substrate as in a patterned sapphire substrate (PSS) to enhance light extraction efficiency of the light emitting diode. However, since the gallium nitride substrate is formed of the same kind of material as that of the epitaxial layer grown thereon, the epitaxial layer and the substrate have substantially the same index of refraction. Thus, even if the pattern is formed on the gallium nitride substrate, since there is no difference in index of refraction between the substrate and the epitaxial layer, the pattern does not cause scattering or refraction of light. Thus, light generated in an active layer reaches a bottom surface of the substrate through the gallium nitride substrate having a relatively high thickness of about 300 μm, thereby causing substantial loss of light in the gallium nitride substrate.
In addition, since the sapphire substrate is electrically non-conductive, the sapphire substrate restricts the structure of the light emitting diode. In recent years, various attempts have been made to develop a technology of manufacturing a vertical type light emitting diode by growing epitaxial layers such as nitride semiconductor layers on a heterogeneous substrate, for example, a sapphire substrate, bonding a support substrate to the epitaxial layers, and separating the heterogeneous substrate from the epitaxial layers by laser lift-off or the like.
Generally, as compared with a typical lateral type light emitting diode in the art, the vertical type light emitting diode includes a p-type semiconductor at a lower portion thereof to secure excellent current spreading performance, and employs a support substrate having higher thermal conductivity than the sapphire substrate to provide excellent heat dissipation.
In addition, an n-type semiconductor layer disposed at an upper portion of the vertical type light emitting diode may be subjected to anisotropic etching such as photo-enhanced chemical (PEC) etching and the like to form a roughened surface on the n-type semiconductor layer, thereby significantly improving light extraction efficiency.
However, in such a vertical type light emitting diode, since the overall thickness (about 4 μm) of the epitaxial layers is very thin as compared with a light emitting area of, for example, 350 μm×350 μm, or 1 mm2, there is severe difficulty in current spreading.
To address such problems, an electrode extension extending from an n-type electrode pad is formed to allow current spreading within the n-type semiconductor layer, or an insulation material is disposed on a p-type electrode at a place corresponding to the n-type electrode pad to prevent direct flow of electric current from the n-type electrode pad to the p-type electrode.
However, these structures also have a limit in prevention of current crowding under the n-type electrode pad and cannot achieve uniform spreading of electric current over a wide light emitting area.